Twin-arm interferometer spectrometers are known to prior art. The Michelson interferometer spectrometer is a representative example. An incident beam of analytical radiation, such as collimated infrared radiation, strikes a beamsplitter and is split into two separate beams, each of which travels down a different optical path or "arm" of the interferometer. One of the beams, known as the reference beam, is directed along a reference path having a fixed optical length while the other beam, known as the test beam, is directed along a test path having a variable optical length. Retroreflecting elements, such as plane mirrors or corner-cubes, return the two beams to the beamsplitter wherein the beams recombine to form a single exit beam. The exit beam is then directed to a sample and thereafter to a suitable radiation detector. The exit beam is then modulated, by varying the optical path length in the test beam, and the detected radiation is converted into a signal which is then analyzed in a known way to determine certain characteristics, such as the spectrum, of the sample. In other designs the sample may be placed in the incident beam or in the test beam, but the same operating principles apply. Other characteristics of the sample, such as its thickness or index of refraction, may also be determined from this basic apparatus.
Because the design relies on the wave interference between the two arm beams of the device, an important factor in determining the quality of this type of interferometer is the degree to which the optical elements in the test arm remain aligned with the optical elements in the reference arm during variation of the path length of the test beam. In most designs, the optical length of the test beam is varied by displacing the retroreflecting element, commonly referred to as the moving mirror, in the test arm longitudinally along the optical axis of the test beam. The maximum resolution attainable with the device is directly related to the maximum path difference, i.e., the maximum longitudinal displacement, that is attained by the displaced retroreflecting element.
Unfortunately, the longitudinal displacement of the moving mirror must be extremely accurate over the entire range of its travel. In most cases, the moving mirror must remain aligned to within a small fraction of the wavelength of light over a longitudinal movement of several centimeters. To achieve this high accuracy over such a long travel, modern high-speed scanning interferometers usually use precision air bearings to support the mirror while complicated electronic control systems insure that the mirror orientation is held constant. Unfortunately, precision air bearings are quite expensive, relatively large, and require a supply of pressurized gas in order to operate. In addition, electronic control systems are difficult to design and may also be relatively large and expensive. Other precision bearings and control circuits have similar drawbacks.
Numerous efforts have been made to avoid the need for precision bearings and electronic alignment control. One approach is to use a tiltable reflector assembly, rather than a longitudinally displaced retroreflector, in the test arm of the interferometer. The tilting reflector assembly consists of a pair of parallel, mutually facing mirrors which, when rotated through an angle, produces a variation in the optical length of a test beam reflecting between them. An example of this is shown in British Patent Application No. 2,171,536A. Another example is shown by Sternberg and James, J. Sci. Instrum., 41, 225-6, 1964.
Designs such as these, which use tiltable reflector assemblies in the test arm to vary the optical path length, have the advantage that they are insensitive to linear movement of the optical elements. Instead, the optical elements are arranged so that only angular displacements of the assembly will change the path length of the test arm. Because longitudinal displacement of the assembly will not alter the path length of the arm, and because rotating bearings are generally easier and less expensive to produce than longitudinal or linear bearings, tiltable reflector assemblies have generally been found to be more stable than other moving mirror designs.
In spite of this, they have been largely ignored by the industrial community, for reasons not known to the inventor. In part, it is believed due to the fact that tilting reflector assemblies require larger, more expensive optical elements than do longitudinal scanning designs because rotating the elements lessens their effective diameter. Also, it is perhaps partly due to a belief that tiltable reflector assemblies are unable to deliver high resolution, because high resolution requires large rotation angles, in order to give a large path difference, and large rotation angles may cause vignetting of the radiation beam. Efforts to attain high resolution using a larger beam diameter are also difficult because increasing the beam diameter by even small amounts requires increasing the size of the associated optical elements by large amounts.